This invention generally relates to modulated carrier communications networks and more specifically to networks which require a data transmitter and receiver to operate synchronously.
Modulated carrier data communications networks convey data between a central site and one or more remote sites over telephone lines or other communications links. The central site typically comprises a digital data transmitter and receiver, such as a digital computer and its associated input/output control system. Each remote site also comprises a digital data transmitter and receiver, such as a peripheral unit.
At the central site, a modulator circuit receives the digital data from the central site transmitter and converts the data to appropriately modulated carrier signals for transmission over a data transmission path to a designated remote site. At the remote site, a demodulator circuit receives the incoming modulated carrier signal and converts it back to its original digital form for use by the data receiver. Similarly, a data transmitter at a remote site supplies data to its modulator for conversion to modulated carrier signals for transmission back to the central site where a demodulator converts the incoming modulated carrier signal back into digital form for the data receivers at the central site. Generally, a modulator circuit and a demodulator circuit at each site are packaged together in a "modem."
A single data transmitter and receiver at a central site may communicate with one or more remote sites, or "drops" over one or more separate communications links. Each communications link couples a central modem and a number of drops. Appropriate circuits at the central site and each remote site enable any one remote site to communicate with the central site at any given time.
In a typical communications network, the central site is connected to each remote site over a transmission link comprising one or two pairs of transmission lines to enable "full-duplex" or "half-duplex" operation, respectively. When a link comprises two pairs of transmission lines for full duplex operation, one pair carries modulated carrier signals to all remote sites while the other pair carries modulated carrier signals from the remote sites to the central site. In a half-duplex operation one pair of transmission lines carries all modulated carrier signals. These "lines" are not merely two pairs of conductors. Rather, they include switching elements, amplifier elements, and other circuit elements as well as conductor elements which are common to telephone lines. All of these elements are subject to various influences which degrade the transmission characteristics of the line and distort the modulated carrier signal. These influences evidence themselves as the following:
1. additive noise including thermal noise, noise due to switching operations and noise due to atmospheric disturbances;
2. harmonic distortion; PA1 3. frequency offsets introduced by telephone company equipment as it performs frequency division multiplexing operations; it is manifested by a difference between the carrier frequencies at the transmitting and receiving modems; PA1 4. phase jitter which is the rapid changing of frequency offset with time; and PA1 5. bandwidth limitations of the telephone transmission link which introduce linear distortion.
There are many schemes for transferring data in a communications network, characterized by any one or a combination of amplitude, phase or frequency modulating techniques. Generally, these techniques are developed assuming a transmission link which is subjected to only one influence, as opposed to real transmission links which are subject to some or all of the above-mentioned influences. Additive noise commonly is selected because it can be analyzed mathematically and is easy to consider from a theoretical standpoint. However, the different sources of line degragation affect the transmission link differently and minimizing the effect of noise does not necessarily minimize the effects of harmonic distortion, frequency offset, phase jitter or the limited bandwidth. Recently, some attempts have been made to optimize transmitter design with respect to noise and phase jitter, but the receiver design has been optimized only with respect to noise.
There are two approaches for solving or devising a modulating and demodulating technique for a data communications network: a mathematical approach and an empirical approach. Mathematical solutions generally do not provide good overall performance. Furthermore, they often produce decision regions and transmission points which are not symmetrical. Without symmetry, separate circuits corresponding to each decision region and corresponding transmission point may be necessary. These approaches, therefore, lead to costly circuits of less than ideal performance.
Empirical designs are based upon various mathematical principles to some degree. However, the consideration of producing an easily implemented design is overriding. Performance thus is often sacrificed in order to obtain symmetry and to simplify the circuits. However, the resulting circuits are affected adversely by moderate additive noise and phase jitter.
Therefore, it is an object of this invention to provide a technique for modulating and demodulating signals in a data communications network in which overall performance is improved.
Still another object of this invention is to provide a technique for modulating and demodulating signals in a data communications network which improves performance with minimal increase in expense.
Still another object of this invention is to provide a modulating and demodulating technique for data communications networks which improves overall network performance in response to distorted signals from degraded transmission lines.